1. Procedures are described for the extraction and partial purification of dog renin, on a large scale, as well as for the acetylation of rat, rabbit, and dog renin.
2. Untreated homologous renin was not antigenic in rat, rabbit, or dog, but the acetylation of homologous renin made it antigenic.
3. Immunization of rats, rabbits, and dogs, with acetylated rat, rabbit, and dog renin, respectively, resulted in each case in the development of anti-renin to the homologous, untreated, as well as to the acetylated renin.
4. The progressive development of antirenin as a result of repeated, subcutaneous injections of acetylated dog renin, in a dog with experimental renal hypertension for more than 6 years, was accompanied by a correspondingly progressive fall of the mean arterial blood pressure to the prehypertensive level. This points up the important part played by the renin-angiotensin mechanism in the maintenance of the hypertension, even in the chronic phase of experimental renal hypertension.